1. Field of the Invention
The present invention relates to a flash unit having a variable illuminating angle mechanism which changes the illuminating angle of a flash unit, and, more particularly, the present invention relates to a variable illuminating angle mechanism for a flash unit which stops the movement of a light emitting body or an illuminating lens within the flash unit from exceeding a normal operating range.
2. Description of the Related Art
When flash photography is performed with a camera, it is necessary for the field angle of the photographic lens and the illuminating angle of the flash unit to overlap. In a conventional camera, to control the overlap of the field angle and the illuminating angle, the camera transmits data representing the focal length of the photographic lens to the flash unit, and the flash unit uses the focal length data to adjust the illuminating angle by causing a light emitting body or an illuminating lens to move. This type of camera is disclosed in Japanese Laid-Open Patent Publication No. JP-A-2-207139, and in Japanese Laid-Open Patent Publication No. JP-A-57-120922.
FIG. 7 is a cross-sectional diagram of a conventional flash unit 71 of the type described above. As shown in FIG. 7, the flash unit 71 includes a motor 73 which rotationally drives a freely rotatable drive shaft 72. A transmission member 74 is in threaded connection to the drive shaft 72, and is also connected to a light emitting body 75. As the motor 73 rotationally drives the drive shaft 72, the transmission member 74 is moved which in turn moves the light emitting body 75. Further, an illuminating window 76 consisting of a Fresnel lens is positioned in the illuminating direction of the light emitting body 75.
A contact 77 is fixed to the lower surface of the transmission member 74, and a contact point 78 located along the drive shaft 72 makes sliding contact with the contact 77. The contact point 78 is connected to a control unit 79, and an output of the control unit 79 is connected to the motor 73. An input terminal of the control unit 79 is connected to components in the camera.
In operation of the flash unit shown in FIG. 7, the control unit 79 calculates a target position of the light emitting body 75 according to focal length information provided from the camera to provide an appropriate illumination angle. Next, the control unit 79 measures the actual position of the light emitting body 75 according to the state of the contact point 78, and the control unit 79 provides a drive output to the motor 73 corresponding to the positional deviation of the target position and the actual position. The motor 73 then rotationally drives the drive shaft 72 according to the output from the control unit 79. The control unit 79 successively detects the position of the transmission member 74 as the light emitting body 75 is driven to the target position, and stops the motor 73 after it has driven the transmission member 74 and the light emitting body 75 to the target position. In the above-described manner, the illuminating angle of the flash unit 71 is adjusted by changing the position of the light emitting body 75 within the flash unit 71.
When, for example, a zoom lens is used, the focal length of the photographic lens frequently changes according to the photographic circumstances. Because of the frequent changes of the focal length when using a zoom lens, a mechanism as shown in FIG. 7 which adapts the illuminating angle to the focal length is advantageous to increase the mobility of photography.
However, when an input of data representing the position of the transmission member 74 is absent due to electrical open circuit or poor connection, the control unit 79 becomes unable to detect the position of the light emitting body 75. In such a state, the control unit 79 drives the motor 73 continuously. Further, because the flash unit 71 impresses a high voltage trigger on the light emitting body 75 at this instant, electrical noise acts on the control unit 79. Since the control unit 79 consists of a microcomputer or the like, the electrical noise causes operations other than the programmed operations to arise (hereinafter referred to as "runaway"), and there is a risk of the motor 73 continuing to rotate.
In the case that the control unit 79 is unable to detect position data of the transmission member 74, it is possible to stop the motor 73 if the motor 73 is caused to rotate for more than a predetermined time. Such a system is disclosed in Japanese Examined Patent Publication JP-B-7-27150.
However, in the case that runaway of the control unit 79 occurs, a stop command by the control unit 79 is not generated, and no stop operation is performed. If no stop operation is performed, the motor 73 continues to rotate, and there is a risk of failure of the variable illuminating angle mechanism. Moreover, when the transmission member 74 reaches the end of the drive shaft 72, and the rotation of the motor 73 is forced to stop (hereinafter referred to as "lock"), a risk of failure of the motor 73 arises.
Generally, as a countermeasure against the kinds of problems described above, a slipping clutch is disposed between the motor 73 and the drive shaft 72 so that the motor 73 does not lock. However, when a slipping clutch is used between the motor 73 and the drive shaft 72, it is necessary to provide a sliding surface with a predetermined static frictional force in order to transmit the drive force to the drive shaft 72 during normal operation. A problem occurs in that a comparatively large sliding frictional force occurs during an abnormality, causing the sliding surface to wear and its durability to become low.
Furthermore, a problem occurs in that the large sliding frictional force acts on both the motor 73 and the drive shaft 72, becoming a new origin of failure in the variable illuminating angle mechanism.
Moreover, a problem occurs in that the necessity of using a slipping clutch between the motor 73 and the drive shaft 72 causes the transmission mechanism to become complicated. As a result, the ease of assembly of the flash unit becomes poor and the size of the flash unit is increased.